The invention relates to communications networks in general. More particularly, the invention relates to a method and apparatus to perform forward error correction in a network such as a long-haul communications network.
Long-haul communication networks are designed to carry information over relatively long distances, typically in the range of 600-10,000 kilometers. Examples of long-haul communications systems include xe2x80x9cunderseaxe2x80x9d or xe2x80x9csubmarinexe2x80x9d systems that carry signals from one continent to another (e.g., North America to Europe). These systems are typically optical systems given the advantages in terms of capacity and reliability.
One problem associated with long-haul communication systems is maintaining the integrity of the data being communicated. All communication systems are susceptible to noise and pulse distortion to some extent. Long-haul communication systems are particular susceptible to noise and pulse distortion given the greater distances over which they carry information.
Forward Error Correction (FEC) is a technique used to help compensate for this distortion. FEC is essentially the incorporation of a suitable code into a data stream, for the detection and correction of data errors without any previously known information. A transmitter receives a data stream and encodes the data stream using an FEC encoder. The FEC encoder generates a code for a block of data, which is appended to the block of data. The transmitter sends the encoded block of data over the network. A receiver receives the encoded block of data and runs it through an FEC decoder. The FEC decoder recovers the code and uses it to detect and correct any errors within the received block of data.
The use of FEC in a system provides xe2x80x9cmargin improvementsxe2x80x9d to the system. The margin improvements can be used to increase amplifier spacing or increase system capacity. In a Wavelength Division Multiplexing (WDM) system, the margin improvement can be used to increase the bit rate of each WDM channel, or decrease the spacing between WDM channels thereby allowing more channels for a given amplifier bandwidth. Consequently, improvements in FEC techniques directly translate into increased capacity for long-haul communication systems. Accordingly, it can be appreciated that a substantial need exists for an enhanced FEC method and apparatus that improves margin requirements and therefore system capacity.
One embodiment of the invention comprises a method and apparatus to perform error correction. A stream of data is encoded using concatenated error correcting codes. The encoded data is communicated over a long-haul transmission system. The encoded data is decoded using the codes and three levels of decoding.